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Liaoningosaurus: perhaps not an ankylosaur

This one goes back several years… with several updates!Xu, Wang and You 2001 described what they thought was a juvenile ankylosaur, Liaoningosaurus paradoxes (Early Cretaceous, Yixian Formation) featuring “a large bony plate (somewhat shell-like) shielding the abdomen.” They tested it against only 13 other taxa and nested it outside the nodosaurs and outside the ankylosaurs…with no taxa between it and Stegosaurus.

Figure 1. Several specimens of Liaoningosaurus crushed flat plus a lateral view based on original holotype tracings.Note the lizard-like sprawling limbs in situ, a product of taphonomic crushing. Like all dinos, this one also had vertical limbs. Only a few small osteoderms are identified.

Xu et al. report:“Diagnosis. An ankylosaurian that differs in having: shell-like ventral armour, trapezoidal sternum with slender and distally pointed posterolateral process and short medial articular margin, and pes more than twice as long as manus.”

Perhaps Xu et al. focused on ankylosaurs and nodosaurs
because all the specimens of Liaoningosaurus I have seen in publications or online (Fig. 1) have been crushed flat, with ribs spread out like ankylosaur ribs. Moreover, the pelvis was very wide, with limbs beneath the ilia, like those in ankylosaurs.

A closer look
(Fig. 1) reveals the ribs would have enclosed a deeper chest, not a wider one, though not as relatively deep as in Stegosaurus. Other more primitive stegosaurs likewise had a shorter, rounder torso cross-section.

Note,
the limbs are preserved sprawling, like those of the horned lizard, Phrynosoma. No dinosaur had sprawling limbs, so it’s okay to bring in both the limbs and the ribs (Fig. 1).

Finally,
basal stegosaurs also have a very broad pelvis with limbs rotating beneath the ilium. Considering how closely ankylosaurs and stegosaurs match each other in so many traits, it is a tribute to the LRT that it recovers them in separate clades, separated by bipedal agile taxa like Lesothosaurusand Heterodontosaurus. It is unlikely that ankylosaurs ever reared up on their hind limbs, but stegosaurs appear to be able to do this.

Osteoderms are rare in Liaoningosaurus,
which is odd for an armored ankylosaur.

There are five digits on the manus
in Liaoningosaurus (Fig. 1) and metacarpal #5 is as long as #4. Unfortunately, ankylosaurs and kin in the LRT lack a preserved manus, but a look through the Princeton Field Guide to Dinosaurs (Paul 2010) finds no similar ankylosaur manus.

Arbour et al. 2014 report,“Examination of the holotype of Liaoningosaurus paradoxus, IVPP V12566, indicates that the ventral “plastron” is better interpreted as epidermal scales, because the broken edges do not reveal any bony histology.” Readers will note that the odd ventral plate (closeup in Fig. 2) does not appear in other Liaoningosaurus specimens (Fig. 1), but they are exposed dorsally.

Figure 2. Liaoningosaurus ventral patch. Note the scales.

The large reptile tree (LRT, 1005 taxa) includes several more ornithischian taxa, though fewer taxa with armor. In the LRT Liaoningosaurus nests between Scutellosaurus and Stegosaurus, several nodes away from the other armored ornithischians,Minmi and Scelidosaurus. No skull traits were tested in Liaoningosaurus due to the low resolution of the available images.

The armored ornithischians (stegosaurs and ankyosaurs) are so similarto one another they are traditionally nested in one clade: Thyreophora. By contrast, the LRT separates ankylosaurs from stegosaurs. Here the few hind limb traits that separate Liaoningosaurus from Scelidosaurus and/or Minmi and ally it with Scutellosaurus and/or Stegosaurus include the following:

Tibia/femur ratio not less than 1:1

Fibula not appressed to tibia

Fibula diameter not > half tibia diameter

Metatarsus not compact

Metatarsal 1 < half metatarsal 3

Metatarsal 1 not > half metatarsal 4

Metatarsals 2 and 3 align beyond p1.1

Pedal 4 length < metatarsal 4

Perhaps better imagery
of the skull and other parts will add to or modify this list and nesting.

The addition of a basal ankylosaur
with these traits would nudge Liaoningosaurus toward the ankylosaurs. In the LRT ankylosaurs were derived from large, armored, lumbering Scelidosaurus. By contrast, the stegosaurs were derived from small, agile Lesothosaurusand Scutellosaurus. So finding a small armored dinosaur with the above list of traits, even if it is a juvenile, should suggest taking a close look at its stegosaur affinities, despite the initial appearance of a wide round horned-lizard-like torso.

PSJi et al. 2016 found fish within the torso (but not restricted to the gut) of a Liaoningosaurus suggesting a fish diet, rather than an herbivorous one.

PPSXu et al. 2001 reported, “Liaoningosaurus has an unusual combination of characters and it might (for example) represent a third ankylosaur lineage.” Perhaps one closer to stegosaurs. Xu et al. 2001 also report, “all manual and pedal unguals claw-shaped.” At present the manual unguals do not appear to be claw-shaped, with the the exception of #3, as in stegosaurs… AND as in ankylosaurs.

When bones are crushed to a single plane, two of their dimensions usually remain unchanged. That’s what I’m looking at.
re: ancestors. Unlikely as it is, if we actually found the exact ancestor of ankylosaurs the cladogram would still nest it as a sister taxon. That’s just how the graphics work.